scratch.js

function compressed(arr) {
  const sortedArr = arr.sort((a, b) => a - b);
  let result = "";
  let isAdding = false;
  let latestLoop = false;
  for (let i = 0; i < sortedArr.length; i++) {
    const resultElement = sortedArr[i];
    const nextResult = sortedArr[i + 1];
    if ( nextResult === undefined) {
      latestLoop = true;
      result += resultElement;
    }
    if (resultElement + 1 === nextResult && !isAdding && !latestLoop) { // the first init
      result += resultElement;
      result += "-";
      isAdding = true;
    }
    if (isAdding && resultElement + 1 !== nextResult && !latestLoop) { // last element
      result += resultElement;
      result += ",";
      isAdding = false;
    }
    if (!isAdding && resultElement + 1 !== nextResult && !latestLoop) {
      result += resultElement;
      result += ",";
    }
  }
  return result;
}

console.log(compressed([3, 2, 1, 5, 6, -1, 10]));